But if your screen is unlocked and open, the decline and answer buttons will show.

For times when your phone is locked, pressing the sleep button on the side of your phone once will stop your iPhone from ringing or vibrating — a second click will decline the call.

The same question has plagued iPhone owners for years: Why can't I always decline an incoming call?

Often the only option given is "slide to answer" — leaving someone trying to avoid a call watching their phone ring until voice mail picks up. Those who have spotted the "decline" button wonder why it isn't always there.

If your screen is unlocked when the call comes in, this is what you'll see:

This mode makes things much more simple. Kim Renfro/INSIDER

But there's a trick. You actually can decline an incoming call when your screen is locked — it just takes a couple more clicks.

Pressing the lock button once will stop your iPhone from ringing or vibrating. And the second click will decline the call. Voilà! But be warned: The incoming caller will know that you've chosen to decline their call, since your voice mail will kick in sooner than expected.

Now go forth into the world with your newfound knowledge and never stare at your ringing phone screen again.

Sign up here to get INSIDER's favorite stories straight to your inbox.